Graft versus host disease (GVHD) is the major complication associated with allogeneic hematopoietic stem cell transplantation, and is characterized by both acute and chronic phases which have different pathophysiological mechanisms. Despite the use of pharmacological strategies which are primarily based on the administration of calcineurin-based agents, both acute and chronic GVHD remain major clinical problems. In preliminary studies, we have identified that signaling through the cannabinoid type 2 receptor (CB2R) mitigates the severity of both acute GVHD and also plays a pivotal role in reducing fibrosis in chronic GVHD. Moreover, we have discovered that that the nonpsychoactive, phytocannabinoid, cannabidiol (CBD) also inhibits GVH reactivity and potentiates adenosine signaling. Based on these studies, our overall hypothesis is that cannabinoids mitigate the severity of both acute and chronic GVHD, and that these anti-inflammatory effects are mechanistically mediated through the CB2R and the adenosine receptor signaling pathways. Studies in Specific Aim 1 will utilize a novel CB2R reporter mouse model that has flanking lox P sites that will allow us to both identify the critical immune cell populations that express the CB2R, and then eliminate these identified cells to formally test a functional role for these cells in acute GVHD biology. We will also employ several clinically relevant pharmacological approaches to determine whether administration of direct and selective CB2R agonists or, alternatively agents that inhibit the degradation of 2-arachidonyl glycerol (2-AG) which is the major endogenous ligand for the CB2R, can reduce the severity of this disease. Studies in Specific Aim 2 will define the role of CBD in mitigating the severity of acute GVHD by using selective pharmacological antagonists and genetic strategies to test the hypothesis that CBD regulates GVHD through the adenosine 2A receptor (A2AR) signaling pathway. We will also examine whether CBD augments the reconstitution of regulatory T cells and stabilizes Foxp3 expression in these cells through adenosine signaling. Experiments in Specific Aim 3 will determine whether cannabinoid signaling prevents the development of chronic GVHD-associated fibrosis which is a hallmark of this disease. Specifically, we will define the critical CB2R expressing immune cell populations which are present during the fibrotic phase of chronic GVHD and use cell-specific Cre-lox deletion approaches to define the functional significance of CB2R expression in identified cells. We will also assess whether either pharmacological administration of direct CB2R agonists, agents that inhibit the degradation of 2-AG, and CBD can prevent chronic GVHD-associated fibrosis. The overall goal of these studies is to define the mechanisms by which cannabinoids modulate both acute and chronic GVHD, and to test clinically relevant strategies that are designed to mitigate these complications in allogeneic hematopoietic stem cell transplant recipients.